In co-pending application Ser. No. 541,699, filed June 21, 1990, there is disclosed a composition of matter having the formula M.sub.2 [Mn.sub.4 (O)(OH)(O.sub.2 CR).sub.2 L.sub.2 ]wherein M is an alkali earth metal selected from magnesium, calcium, strontium, barium or mixtures thereof, R is hydrogen or a hydrocarbyl group, and L is a ligand having the formula: ##STR2## These compounds have been shown to have a core structure of 4 manganese atoms which are bridged by oxo and hydroxo groups and, hence, they are referred to as oxo (hydroxo) bridged tetranuclear manganese compounds.